It seems like a huge roster... eighteen characters are a really huge number. I've never seen so many together since FF6.
Anyway, in my opinion, this could really be dangerous to your game, since you should have to develop not two or three but eighteen different stories, eighteen different fighting styles, eighteen different characters with passions, fears, dreams and so on.
You should be a great writer to set everything on the right path.
I'm generally for parties composed by a maximum of three characters. In my games, main characters are a number between three and five, plus various minor NPCs. I've never had more than seven playable characters, for the reasons I explained before.
Creating the whole story could be very painful and difficult with such a number of chars.
Good luck and best wishes